The role of the structural features (Cu–Cu distance) of Cu2O in C–C coupling in CO2RRs with H2O under solar irradiation. A DFT study for elucidating the reaction mechanism

In photochemical (and photoelectrochemical) coprocessing of CO 2 and H 2 O, the control of the selectivity plays a fundamental role for maximising the light (and Faradaic) efficiency towards a product. In fact, a selective process would avoid post-reaction separation operations, saving energy and resources. Recently, we have shown that different frustules (the siliceous skeleton of microalgae diatoms ) induce the formation of distinct facets ({111} octahedral or {110} rhombohedral) of Cu 2 O, which are characterized by a different Cu–Cu distance (see text). Notably, each facet promotes the formation of different CO2RPs. In particular, {111} affords methanol while {110} produces C 2+ species. These findings suggested that the Cu–Cu distance in the Cu 2 O facets may play a key role in C–C coupling, additionally opening the question about single-site or multi-site catalysis in CO2RRs. To contribute to clarifying the role of the intermetallic distance in the Cu 2 O-driven photocatalysis, we have modelled by DFT calculations the CO 2 reduction under PCET (Proton Coupled to Electron Transfer) conditions compatible with the mild reaction conditions (0.1 MPa pressure and maximum temperature of 45 °C) used in our photochemical experiments. The results of the present study shed light on three key points: i. the {110} rhombohedral facet of Cu 2 O shows the most suited Cu–Cu distance ( ca. 260 pm) for C–C coupling with respect to {111} octahedral or {100} cubic facets (Cu–Cu distance of 370 and 417 pm, respectively), ii. the multi-site mechanism is energetically preferred to the single-site catalysis for C–C coupling; iii. engineering the surface structure of the photo(electro)catalyst is essential for a high selectivity towards a given CO2RP, either a C 1 or a C n species, determining the process energy efficiency.